With increased demand being placed on existing optical fiber facilities, lightwave communications providers are looking for ways to increase the usable bandwidth available for customers from existing fiber without installing additional fibers. Lightwave communication systems depend on optical fiber to transport the lightwave signals from one location to another in the system.
Optical fiber, both single mode and multimode, has modal and chromatic dispersion parameters which result from material and waveguide characteristics of the fiber. Chromatic dispersion causes lightwaves at one wavelength to travel at a different velocity through the optical fiber than lightwaves at another wavelength. Thus, for example, a short pulse input to one end of the fiber emerges from the far end as a broader pulse. Pulse broadening effects and, therefore, dispersion limit the speed at which information can be carried through an optical fiber.
Several solutions have been proposed to avoid or at least counter the effects of dispersion. These solutions include wavelength-division multiplexing techniques and dispersion compensation techniques. The former technique tends to avoid the dispersion problem by offering a plurality of narrowband channels at specified wavelengths wherein each channel affords a moderate bandwidth capability. Because each channel is confined to a narrow bandwidth, the lightwave signals in a particular channel experience a somewhat uniform dispersive effect over that channel.
On the other hand, dispersion compensation or equalization techniques attempt to counter or undo the effects of dispersion. One technique described in U.S. Pat. No. 4,261,639 involves the interconnection of two optical fibers having appropriate lengths and having opposite group velocity dispersion characteristics so that the total dispersion in one fiber is substantially matched and canceled by the total dispersion in the connected fiber. While this technique offers a possible solution to the dispersion problem, it is impractical (1) because the length of compensating (opposite dispersion) fiber may be very long on the order of the length of the existing installed fiber and (2) because there may be insufficient fiber available having the appropriate dispersion of opposite sign to dispersion of the existing installed fiber.
In order to avoid the use of dispersion compensating fibers as described above, reflective and transmissive Fabry-Perot etalon structures have been proposed for providing optical equalization. For a discussion of these structures, see J. of Lightwave Technology, Vol. 8, No. 5, pp. 649-59 (1990). Adaptive control via a feedback loop is demonstrated for the structures. It is believed that the hardware complexity of these optical equalization structures together with the need to provide substantially continuous monitoring and tuning of the etalon significantly affect the commercial attractiveness of such structures.